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Re: [PATCH] xfs_file_last_byte() needs to acquire ilock

To: Felix Blyakher <felixb@xxxxxxx>
Subject: Re: [PATCH] xfs_file_last_byte() needs to acquire ilock
From: Lachlan McIlroy <lmcilroy@xxxxxxxxxx>
Date: Fri, 24 Apr 2009 01:07:33 -0400 (EDT)
Cc: xfs@xxxxxxxxxxx
In-reply-to: <869141559.5581240549433363.JavaMail.root@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
Reply-to: Lachlan McIlroy <lmcilroy@xxxxxxxxxx>
----- "Felix Blyakher" <felixb@xxxxxxx> wrote:

> On Apr 23, 2009, at 9:18 PM, Lachlan McIlroy wrote:
> 
> > We had some systems crash with this stack:
> >
> > [<a00000010000cb20>] ia64_leave_kernel+0x0/0x280
> > [<a00000021291ca00>] xfs_bmbt_get_startoff+0x0/0x20 [xfs]
> > [<a0000002129080b0>] xfs_bmap_last_offset+0x210/0x280 [xfs]
> > [<a00000021295b010>] xfs_file_last_byte+0x70/0x1a0 [xfs]
> > [<a00000021295b200>] xfs_itruncate_start+0xc0/0x1a0 [xfs]
> > [<a0000002129935f0>] xfs_inactive_free_eofblocks+0x290/0x460 [xfs]
> > [<a000000212998fb0>] xfs_release+0x1b0/0x240 [xfs]
> > [<a0000002129ad930>] xfs_file_release+0x70/0xa0 [xfs]
> > [<a000000100162ea0>] __fput+0x1a0/0x420
> > [<a000000100163160>] fput+0x40/0x60
> >
> > The problem here is that xfs_file_last_byte() does not acquire the
> > inode lock and can therefore race with another thread that is  
> > modifying
> > the extext list.  While xfs_bmap_last_offset() is trying to lookup
> > what was the last extent some extents were merged and the extent
> list
> > shrunk so the index we lookup is now beyond the end of the extent
> list
> > and potentially in a freed buffer.
> >
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index e7ae08d..cf62d9d 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -1258,8 +1258,10 @@ xfs_file_last_byte(
> >      * necessary.
> >      */
> >     if (ip->i_df.if_flags & XFS_IFEXTENTS) {
> > +           xfs_ilock(ip, XFS_ILOCK_SHARED);
> >             error = xfs_bmap_last_offset(NULL, ip, &last_block,
> >                     XFS_DATA_FORK);
> > +           xfs_iunlock(ip, XFS_ILOCK_SHARED);
> >             if (error) {
> >                     last_block = 0;
> >             }
> 
> My understanding from the original patch was that this is one part
> of the fix, and the other was the following change:
> 
> @@ -3206,6 +3208,8 @@ xfs_bmap_del_extent(
>                   */
>                  XFS_BMAP_TRACE_DELETE("3", ip, idx, 1, whichfork);
>                  xfs_iext_remove(ifp, idx, 1);
> +               if (idx >= (ifp->if_bytes /  
> (uint)sizeof(xfs_bmbt_rec_t)))
> +                       idx--;
>                  ifp->if_lastex = idx;
>                  if (delay)
>                          break;
> 
> You don't think it's still needed, do you?
Yes, I do think it is still needed.  While it is related to the
other locking patch it fixes a separate problem.  The above change
(and the rest of the associated changes in the patch) ensure that
we don't explicitly index beyond the end of the extent map by having
a stale value in if_lastex.

> 
> Felix
> 
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